You might like to read this and associated links:
Disruption Compensation | Eurostar
Like air travel, you have rights to compensation under EU regulation if your train experiences delays or is cancelled. Now how much can a rail provider escape or reduce their responsibilities under the provisions due to "exceptional circumstances" is unknown (or at least I don't know).
Either way, if trains get cancelled, whilst you generally have some form of
carte blanche for your replacement service, don't forget that there's likely going to be a good couple of hundred people who are in the same situation, overwhelming the same staff.
I guess, if you choose to travel in Eurostar Standard class, it might pay to shop a bit beforehand for a good sandwich and drink, or whatever. Note that once you pass into the secure area of St Pancras, there is basically only one cafe selling some toasties and other standard cafe bric-a-brac, then you have whatever Eurostar offer on board. So for more variety (e.g. sushi, etc.), best to find a place and buy up beforehand.